1. Field of the Invention
This invention relates to a system and method for removal of solids, pathogens, nitrogen, and phosphorus from municipal and agricultural wastewater.
2. Description of the Related Art
Municipal and agricultural waste disposal is a major problem. For agricultural animals, the animals are confined in high densities and lack functional and sustainable treatment systems. The liquid wastes are generally treated in large anaerobic lagoons with intermittent disposal through land applications (Stith, P. and Warrick, J., Boss Hog: North Carolina's pork revolution, The News & Observer, 1-3, Feb. 19-26, 1995; USEPA, Proposed regulations to address water pollution from concentrated animal feeding operations, EPA 833-F-00-016, Januray. 2001, Office of Water, Washington, D.C. 20460). This system was developed in the early and mid 20th century prior to the current trend in high concentrated livestock operations. One of the main, problems in sustainability is the imbalance of nitrogen (N) and phosphorus (P) applied to land (USEPA, supra; Cochran et al., Dollars and Sense: An economic analysis of alternative hog waste management technologies, Environmental Defense, Washington, D.C., 2000). Nutrients in manure are not present in the same proportion needed by crops, and when manure is applied based on a crop's nitrogen requirement excessive phosphorus is applied resulting in phosphorus accumulation in soil, phosphorus runoff, and eutrophication of surface waters (Heathwaite et al., A conceptual approach for integrating phosphorus and nitrogen management at watershed scales, J. Environ. Qual., Volume 29, 158-166, 2000; Sharpley et al., Practical and innovative measures for the control of agricultural phosphorus losses to water: An overview, J. Environ. Qual, Volume 29, 1-9, 2000; Edwards and Daniel, Environmental Impacts of On-Farm Poultry Waste Disposal-A Review, Bioresource Technology, Volume 41, 9-33, 1992).
The change from small individual animal production operations to large, confined, commercial enterprises has caused many problems for the animal production industry including emission of ammonia (NH3) from lagoons. It may be anticipated that about 50-80% of the nitrogen (N) entering animal lagoons will escape to the atmosphere through NH3 volatization (Miner and Hazen, Transportation and application of organic wastes to land, In: Soils for Management of organic Wastes and Waste Waters, 379-425, eds. L. F. Elliot and F. J. Stevenson, Madison Wis.: ASA/CSSA/SSSA; Barrington and Moreno, Swine manure nitrogen conservation using Sphagnum moss, J. Environ. Quality, Volume 24, 603-607, 1995; Braum et al., Nitrogen losses from a liquid dairy manure management system, In: Agron. Abstracts, Madison, Wis.: ASA, 1997). Biological removal of nitrogen through the process of nitrification and denitrification is regarded as the most efficient and economically feasible method available for removal of nitrogen from wastewaters. The effectiveness of the biological nitrogen removal process depends on the ability of nitrifying organisms to oxidize ammonium ions (NH4+) to nitrite (NO2−) and nitrate (NO3−). Subsequent reduction to molecular nitrogen, denitrification, may be essential as well if one desires to reduce total nitrogen as well as ammonia nitrogen. This step is rapid with available carbonaceous substrate and an anaerobic environment, conditions which are typically found in farm settings in constructed wetlands or liquid manure storage units. The reaction rate of nitrification is extremely low compared to that of denitrification, so that nitrification normally will be a rate limiting step in the biological nitrogen removal process (Vanotti and Hunt, Transactions of the ASAE, Volume 43(2), 405-413, 2000).
The basic problem related to nitrification in wastewaters with a high content of organic carbon is the low growth rate of the nitrifying bacteria; the generation time of these microorganisms is about 15 hours. Compared to heterotrophic microorganisms, which have generation times of 20 to 40 minutes, the nitrifiers compete poorly for limited oxygen and nutrients and tend to be overgrown or washed out of reactors (Figueroa and Silverstein, Water Environ. Res., Volume 64(5), 728-733,1992; Wijffels et al., Possibilities of nitrification with immobilized cells in wastewater treatment Model or practical systems, Wat. Sci. Tech., Volume 27(5-6), 233-240, 1993). The nitrification of lagoon swine wastewater is an especially difficult process because of the very low numbers of Nitrosomonas and Nitrobacter usually found after anaerobic treatment (Blouin et al., Nitrification of swine waste, Canadian J. Microbiol., Volume 36, 273-278,1990). Even when the oxygen supply is plentiful, an adaptation period is needed to reach a minimum bacteria concentration for effective nitrification. Recycling surplus activated sludge in an aerobic reactor or long hydraulic retention time (HRT) is required to retain slow growing autotrophic nitrifiers. Unfortunately, in the absence of enriched nitrifying populations, aerobic treatment of lagoons can potentially add to problems by stripping ammonia into the atmosphere, particularly if uncontrolled or excessive flow rates of air are used (Burton, A review of the strategies in the aerobic treatment of pig slurry: Purpose, theory, and method, J. Agric. Eng. Res., Volume 53, 249-272, 1992).
The efficiency of the nitrification process can be increased by increasing the nitrifiers' retention time independent from the wastewater retention time (Wijffels et al, 1992; supra). In most cases, this is done by immobilization of nitrifiers. One advantage of this technology is that increased wastewater flow is possible with minimal washout of immobilized bacteria. Immobilization has been widely used in wastewater treatment applications by taking advantage of spontaneous attachment of cells to the surface of inert support materials. Applications of attached growth for treatment of swine wastewater have been developed by Ciaccolini et al. (Tests for nitrification of effluents from anaerobic digestion of swine wastes, with recovery of fertilizers for agricultural use, Acqua-Aria, Volume 2, 145-154, 1984) and St.-Arnaud et al. (Microbiological aspects of ammonia oxidation of swine waste, Canadian J. Microbiol, Volume 37, 918-923, 1991) who reported higher nitrification rates compared to systems where microorganisms were in suspension.
Managing agricultural sources of phosphorus and nitrogen at the watershed scale in order to reduce their impact on water quality requires a balanced and holistic approach (Heathwaite et al., J. Environ. Qual., Volume 29, 158-166, 2000). In the past, most emphasis has been placed on nitrogen management to ameliorate nitrate losses to ground water. While the high solubility and mobility of nitrate within agricultural systems may justify this emphasis, such bias ignores other critical elements, notably phosphorus.
Advances in biotechnology using immobilization technology have shown that higher nitrification efficiencies are possible through the entrapment of cells in polymer gels, a common technique in drug manufacturing and food processing. The successful application for nitrification treatment of municipal wastewater has been demonstrated using both natural polymers such as calcium alginate (Lewandowski et al., Nitrification and autotrophic denitrification in calcium alginate beads, Wat. Sci. Tech., Volume 19, 175-182, 1987) and synthetic polymers such as polyvinyl alcohol, PVA (Furukawa et al., Preparation of marine nitrifying sludge, J. Ferment. Bioeng., Volume 77(4), 413-418, 1994). Pellets made of synthetic polymers are superior to natural polymers in terms of strength and durability; their estimated life span is about 10 years. These characteristics are very important in long-term biotreatment operations. For this reason, synthetic polymer pellets are preferred for pilot- and plant-scale purposes. There are currently several full-scale municipal wastewater treatment plants using this technology in Japan (Takeshima et al., Pegasus: An innovative high-rate BOD and nitrogen removal process for municipal wastewater, IN: Proc. 66th Annual Water Environment Federation Conf., 173-181, Anaheim, Calif.:WEF, 1993). The nitrifiers are entrapped in 3- to 5-mm polymer pellets permeable to NH4+, oxygen, and carbon dioxide needed by these microorganisms, resulting in a fast and efficient removal of nitrogen. Tanaka et al. (Kinetics of nitrification using fluidized bed reactor with attached growth, Biotechnol. Bioeng., Volume 23, 1686-1702, 1981) reported nitrification rates three times higher than those of the conventional activated sludge process. Previous work with nitrifying pellets has been done exclusively in municipal-type systems where typical NH4+concentrations are about 30 mg N L−1 and BOD5<90 mg L−1.
Phosphorus inputs accelerate eutrophication when it runs off into fresh water and has been identified as a major cause of impaired water quality (Sharpley et al., 2000, supra). Eutrophication restricts water use for fisheries, recreation, industry, and drinking due to the increased growth of undesirable algae and aquatic weeds and resulting oxygen shortages caused by their death and decomposition. Also many drinking water supplies throughout the world experience periodic massive surface blooms of cyanobacteria. These blooms contribute to a wide range of water-related problems including summer fish kills, unpalatability of drinking water, and formation of trihalomethane during water chlorination. Consumption of cyanobacteria blooms or water-soluble neuro- and hepatoxins released when these blooms die can kill livestock and may pose a serious health hazard to humans. Recent outbreaks of the dinoflagellate Pfiesteria piscicida in near-shore waters of the eastern United States also may be influenced by nutrient enrichment. Although the direct cause of these outbreaks is unclear, the scientific consensus is that excessive nutrient loading helps create an environment rich in microbial prey and organic matter that Pfiesteria and menhaden (target fish) use as a food supply. In the long-term, decreases in nutrient loading will reduce eutrophication and will likely lower the risk of toxic outbreaks of Pfiesteria-like dinoflagellates and other harmful algal blooms. These outbreaks and awareness of eutrophication have increased the need for solutions to phosphorus run-off.
Past research efforts on phosphorus removal from wastewater using chemical precipitation have been frustrating due to the large chemical demand and limited value of by-products such as alum sludge, or because of the large chemical demand and huge losses of ammonia at the high pH that is required to precipitate phosphorus with calcium (Ca) and magnesium (Mg) salts (Westerman and Bicudo, Tangential flow separation and chemical enhancement to recover swine manure solids and phosphorus, ASAE Paper No.98-4114, St. Joseph, Mich.: ASAE, 1998); Loehr et al., Development and demonstration of nutrient removal from animal wastes, Environmental Protection Technology Series, Report EPA-R2-73-095, Washington, D.C.: EPA, 1973). Other methods used for phosphorus removal include flocculation and sedimentation of solids using polymer addition, ozonation, mixing, aeration, and filtration (See U.S. Pat. No. 6,193,889 to Teran et al). U.S. Pat. No. 6,153,094 to Craig et al. teaches the addition of calcium carbonate in the form of crushed limestone to form calcium phosphate mineral. The patent also teaches adsorbing phosphorus onto iron oxy-hydroxides under acidic conditions.
Continuing efforts are being made to improve agricultural, animal, and municipal waste treatment methods and apparatus. U.S. Pat. No. 5,472,472 and U.S. Pat. No. 5,078,882 (Northrup) disclose a process for the transformation of animal waste wherein solids are precipitated in a solids reactor, the treated slurry is aerobically and anaerobically treated to form an active biomass. The aqueous slurry containing bioconverted phosphorus is passed into a polishing ecoreactor zone wherein at least a portion of the slurry is converted to a beneficial humus material. In operation the system requires numerous chemical feeds and a series of wetland cells comprising microorganisms, animals, and plants. See also U.S. Pat. Nos. 4,348,285 and 4,432,869 (Groeneweg et al); U.S. Pat. No. 5,627,069 to Powlen; U.S. Pat. No. 5,135,659 to Wartanessian; and U.S. Pat. No. 5,200,082 to Olsen et al (relating to pesticide residues); U.S. Pat. No. 5,470,476 to Taboga; and U.S. Pat. No. 5,545,560 to Chang.
U.S. Pat. No. 6,177,077 (Lee et al.) and U.S. Pat. No. 6,200,469 (Wallace) both relate to the removal of nitrogen and phosphorus from wastewater wherein the phosphate is removed using microorganism in aerobic tanks which absorb the phosphorus released from denitrified wastewater. See also U.S. Pat. No. 6,113,788 to Molof et al., U.S. Pat. No. 6,117,323 to Haggerty; U.S. Pat. No. 6,139,743 to Park et al.
There is concern about the introduction and spread of diseases through wastewater. For example, there is great concern about the spread of Foot and Mouth Disease in countries throughout the world. Major programs are in place at present in countries free of Foot and Mouth Disease to prevent the introduction or spread of the disease. The Irish Agriculture and Food Development Authority (Teagasc) implemented a 12-point Foot and Mouth Disease protection plan including restrictions in liquid manure spreading on fields allowing only emergency spreading when manure storage tanks are likely to overflow. If the disease is introduced, it could be spread as an aerosol during liquid manure spreading. The virus can persist in aerosol form for long periods. It is estimated that sufficient virus to initiate infection can be windborne as far as 100 km (Blood, D.C., Radostits, O. M., and Henderson, J. A., Veterinary Medicine, 6th addition, pages 733-737, 1983. Bailliere Tindall, London, U.K.). The virus is resistant to common disinfectants and the usual storage practices. But it is particularly susceptible to changes in pH away from neutral, or to heat treatment using autoclaving under pressure. Liquid swine manure normally has a pH of about 6 to 8, and the Foot and Mouth Disease virus can survive in this pH range. A shift in the pH in either direction below 5 and above 9 makes conditions for survival less favorable. Thus, infectivity of the Foot and Mouth Disease virus may be effectively destroyed by chemicals such as acids and alkalis (Callis, J., and Gregg, D., Foot-and-mouth disease in cattle, pages 437-439, 1986. In J. L. Howard (ed.), Current Veterinary Therapy 3. W. B. Saunders Company. Philadelphia, Pa.). Unfortunately, liquid swine manure contains inherent buffers, mainly carbonates and ammonia, that prevent changes in pH except when large amounts of chemicals are used. In addition to the large chemical need, addition of acid to liquid manure gives a sudden release of hydrogen sulfide and risk of gas poisoning. On the other hand, increase of pH 9 with the addition of alkali chemicals such as calcium hydroxide (lime) or sodium hydroxide is prevented by ammonia equilibrium. This means that the alkali is used to convert ammonia into gas form before effective increase of pH above 9 is achieved. Ammonia volatilization from animal facilities is an environmental problem in and of itself.
While various systems have been developed for treating wastewater for the removal of solids, pathogens, nitrogen, and phosphorus; there still remains a need in the art for a more effective wastewater treatment system. The present invention, different from prior art systems, provides a system which requires minimal chemical addition and at least reduces problems of ammonia emissions during separation of phosphorus from the aqueous phase, and recovers phosphorus in a reusable form. The process also enables precision control of the nitrogen:phosphorus ratio of the treated waste to desired levels to solve problems of phosphorus accumulation in soil or remediation of contaminated spray fields.